The invention concerns a drive mechanism for a magnetic stripe reader-encoder head or print head carriage assembly which is movable along at least one linear guide rod or track by a drive band or cable. The cable is mounted on a drive pulley positioned in the frame at one side of the carriage assembly and a deflecting pulley positioned in the frame at the other side of the carriage. The drive band or cable is driven by a drive unit, consisting of a drive motor and power transmission connected to the drive pulley.
In such mechanisms, the drive unit is generally fixed to the frame and the deflecting pulley is flexibly guided for generating and maintaining the band or cable tension and so that tensile stresses to which the band may be subjected by temperature fluctuations or prolonged operation are simultaneously compensated for.
Very stringent constant speed requirements have to be observed for magnetic head carriages for recording and sensing signals on magnetic records. Even the print quality of printers depends to a considerable degree on the accuracy of the incremental motion of the type carriage. For instance, DIN (German Industrial Standard) 32 744 specifies bit spacings of 0.121 mm for magnetic stripes at a permissible tolerance of .+-.5%, which corresponds to an accuracy of 0.0061 mm. Hitherto, an accuracy of that order has been unobtainable with conventional arrangements. This is mainly due to the fact that the carriage guide is adversely affected, for instance, by the incremental motions of the carriage or by oscillations produced by power transmission gears, which oscillations are transferred to the frame, or to the further fact that the compensating movements of the deflecting pulley, acting as a tensioning pulley, impair the speed constancy of the carriage. Compensating movements are performed, for example, in conjunction with type carrier impact motions or uneven paths, as are encountered by the magnetic head as it moves across bookkeeping journals, savings books, etc.